This invention relates to a supercalender and more particularly to a supercalender for smoothing the opposite surfaces of a coated paper sheet.
The supercalender, usually, comprises hard rolls (metal rolls) and elastic rolls (e.g., cotton filled rolls) alternately arranged in multi-stage, wherein the roll in the lowermost stage is driven by a drive motor for rotation, successively driving the rolls thereabove by friction. A paper sheet, when passing through the nips downwardly from the top roll, is subjected to pressure and friction whereby its opposite surfaces are finished smooth. The reason why the usual supercalender is vertically arranged in this manner is associated with an intention of effectively utilizing the accumulative downward load of rolls for smoothing purpose with each roll loading the roll immediately therebelow by its own weight.
The usual supercalender arranged vertically in this manner is used for smoothing various papers such as coated paper for printing, color developing paper for pressure-sensitive copy paper, and heat-sensitive recording paper, but has an inevitable drawback that if a mark is produced on an elastic roll during operation, the roll must be removed for repair, so that the operation must of necessity be stopped once. Where the supercalender is incorporated in a coater on an in-line basis, the operation of the coater will have to be stopped also, greatly influencing the operability. In this connection, the elastic rolls are very soft since they are made of cotton, wool, synthetic fibers, paper, asbestos, rubber, plastics or the like, and they can be easily marked if the paper is wrinkled or broken. The present situation is that even if operated with care, they have to be repaired after usually about 4-5 days of operation. If the roll mark is left as it is, this will result in the mark being reproduced on the paper during passage, thus not only detracting from the quality of the paper, but also enlarging the roll mark.
Another drawback of the usual supercalender is that it is unsuitable where a smoothing treatment under low pressure is desired. More particularly, in the case of the vertical type, since the weight of each roll is imposed on the roll immediately therebelow, the nip pressure is equal at least to the weight of the rolls. Thus, it is not suitable where a smoothing treatment is required under pressure less than the weight of the rolls.
The primary object of the invention is to provide an improved supercalender which is capable of continuous operation even if a mark is produced on an elastic roll during operation, without having to stop the operation to repair the same.
Another object of the invention is to provide an improved supercalender which is also capable of performing a smoothing operation under desired conditions at will even in cases where smoothing treatment must be performed under low nip pressure.
Other objects and advantages of the invention will be apparent from the following descriptions.